1. Field of the Invention
This invention relates to a method for the production of an absorbent resin. More particularly, it relates to a method for the production of an absorbent resin having a high absorption ratio, having a small water-soluble component, and having excellent in stability to resist the effect of aging.
2. Description of the Prior Art
The absorbent resins have heretofore found utility in various absorbent materials such as paper diapers (or disposable diapers), sanitary articles, and water-retaining agents for soil. The absorbent resins of this type which have been known to the art include hydrolyzed starch-acrylonitrile graft copolymer, neutralized starch-acrylic acid graft copolymer, cross-linked acrylic acid or acrylate polymer, partially cross-linked polyethylene oxide, and cross-linked carboxymethyl cellulose, for example.
In the cross-linked polymers mentioned above, those cross-linked polymers which are obtained by copolymerizing acrylic acid and other water-soluble monoethylenically unsaturated monomers in the presence of a cross-linking monomer form the mainstream of absorbent resins because they use inexpensive raw materials, excel in absorption characteristics, and have no possibility of putrefaction (JP-A-60-24807 and U.S. Pat. No. 4,351,922).
These absorbent resins, however, are generally such that the content of water-soluble components (which are soluble in water) increases proportionally as their absorption ratios rise. When an absorbent resin having a high water-soluble component content is used for a long time as in a diaper, for example, it has the problem of forming a ropy surface on the diaper or the problem of rendering the diaper leaky because the absorbent impairs the perviousness of the diaper to liquid so much that it degrades the practical absorbency of the diaper and the diffusibility of liquid in the diaper. When the amount of a cross-linking agent to be used in the absorbent resin is increased for the purpose of proportionally decreasing the content of water-soluble components therein, the absorption ratio of the absorbent resin is lowered and the range of uses found for the absorbent resin is limited.
In addition to the problem of the content of water-soluble components mentioned above, the absorbent resins have the problem of poor stability arising from the ability of a swelled gel of resin to withstand the effect of aging. When an absorbent resin is used as in a diaper, for example, the swelled gel which the absorbent resin produces on absorbing urine deteriorates (fluidization of the gel) with the elapse of time and eventually leads to a reduction of the absorption characteristics. Likewise, when an absorbent resin is used for a long time in agronomic operations, there is the possibility of it undergoing degradation and decomposition.
As a means for preventing the swelled gel of absorbent resin from undergoing degradation and decomposition, methods which rely on the incorporation of an oxygen-containing reducing inorganic salt or a radical chain inhibitor in the absorbent resin (JP-A 63-118375 and JP-A 63-152667), a method which cause an oxidizing agent to be contained in the absorbent resin (JP-A 63-153060), and a method which causes a sulfur-containing reducing agent to be contained in the absorbent resin (JP-A 63-272349) have been known to the art. However, these methods, invariably necessitate the use of additives in the absorbent resin for the sake of limiting deterioration of the absorbent resin. The use of such additives in the absorbent resin is not always desirable in terms of safety in the light of the fact that the absorbent resin may possibly be used in sanitary articles.
Another method has been proposed which enables the absorbent resin to acquire improved stability to withstand the effect of aging by using the cross-linking agent in an increased amount thereby heightening the cross-linking density of the absorbent resin and enhancing the gel strength. Imparting sufficient stability to the absorbent resin to resist the effect of aging has the disadvantage of extremely lowering the absorption ratio of the absorbent resin because of the high degree of cross-linking.
An object of this invention, therefore, is to provide a novel method for the production of an absorbent resin.
Another object of this invention is to provide a method for the production of an absorbent resin which has a high absorption ratio, contains a water-soluble component only in a small proportion, and excels in stability to withstand the effect of aging.